The present invention relates generally to compositions having a nickel aluminide base for use in high temperature applications. More specifically, it relates to a rapidly solidified tri-nickel aluminide which has improved ductility based on a partial substitution of iron and niobium in the base alloy.
It is known that polycrystalline tri-nickel aluminide castings exhibit properties of extreme brittleness, low strength and poor ductility at room temperature. It is also known that the rapidly solidified tri-nickel aluminide alloy in the absence of low concentrations of boron also exhibits similar properties at room temperature.
The single crystal tri-nickel aluminide in certain orientations does display a favorable combination of properties at room temperature including significant ductility. However, the polycrystalline material which is conventionally formed by known processes, including rapid solidification processing, does not display the desirable properties of the single crystal material and, although potentially useful as a high temperature structural material, has not found extensive use in this application because of the poor properties of the material at room temperature.
For example, it is known that trinickel aluminide has good physical properties at temperatures above 1000.degree. F. and could be employed, for example, in jet engines as component parts for use at operating or higher temperatures. However, if the material does not have favorable properties at room temperature and below, the part formed of the aluminide may break when subjected to stress at the lower temperatures at which the part would be maintained prior to starting the engine and prior to operating the engine at the higher temperatures.
Alloys having a tri-nickel aluminide base are among the group of alloys known as heat-resisting alloys or superalloys. These alloys are intended for very high temperature service where relatively high stresses (tensile, thermal, vibratory and shock) are encountered and where oxidation resistance is frequently required. The nickel aluminide has favorable strength-to-weight ratios for use in aircraft at elevated temperatures and also has favorable oxidation resistance. Various efforts have been made to improve the lack of ductility at lower temperatures.
Accordingly, what has been sought in the field of superalloys is an alloy composition which displays favorable stress resistant properties not only at the elevated temperatures at which it may be used as, for example, in a jet engine but also a practical, desirable and useful set of properties at the lower temperatures to which the engine is subjected in storage and in mounting and starting operations. For example, it is well known that an engine may be subjected to severe sub-freezing temperatures while standing on an airfield or runway prior to starting the engine.
Significant efforts have been made toward producing a tri-nickel aluminide and similar superalloys which may be useful over a wide range of temperatures and which are adapted to withstand the stress to which the articles made from the material may be subjected in normal operations over such a wide range of temperatures. Some such efforts have been successful. For example, U.S. Pat. No. 4,478,791, assigned to the same assignee as the subject application teaches a method by which a significant measure of ductility can be imparted to a tri-nickel aluminide base metal at room temperature to overcome the brittleness of this material.
Also, copending application of the same inventors of the subject application, Ser. Nos. 647,327; 647,326; 647,328; 647,877 and 647,879, filed Sept. 4, 1984, teaches methods by which the composition and methods of U.S. Pat. No. 4,478,791may be improved.
The subject application presents a method and composition for incorporating improvements in the properties of a tri-nickel aluminide over the composition of the U.S. Pat. No. 4,478,791.
Also, copending application Ser. No. 647,328, filed Sept. 4, 1984, teaches a composition and method for improving the properties of nickel aluminide and involves the incorporation of iron in the nickel aluminide as a partial substituent for both nickel and aluminum. The subject application is an improvement over the teaching of the 647,328 application. Each of the applications enumerated above and the 4,478,781 patent are incorporated herein by reference.